In hybrid vehicles and electric vehicles, batteries using lithium-ion or nickel-metal hydride technology are used, said batteries having a large number of electrochemical battery cells connected in series. A battery management system is used to monitor the battery and is intended to ensure both monitoring of safety and a service life which is as long as possible. For this purpose, the voltage of each individual battery cell is measured together with the battery current and the battery temperature, and a state estimation (for example of the state of charge or of the state of aging of the battery) is made. In order to maximize the service life, it is helpful to know the maximum capacity of the battery at a given time, that is to say the maximum amount of electrical power which can be output or taken up. If this capacity is exceeded, the aging of the battery can be greatly accelerated.
Also during a charging process of the battery, the battery management system also continuously monitors significant parameters of the battery in order to avoid damage to individual battery cells or to the entire battery. FIG. 1 shows a typical time profile of a charging current I and of a cell voltage of a battery cell U during a charging process, known from the prior art, of a lithium-ion battery. In a first phase P1, referred to as the CC (constant current) phase, the battery is charged with a constant current, with the result that the cell voltage of a battery cell increases. From the point when a predetermined limiting voltage is reached, the battery is recharged in a second phase P2, referred to as the CV (constant voltage) phase at a constant voltage whose value corresponds, for example, to a cell voltage of 4.1 V, and is below a critical maximum cell voltage (switch-off limit Umax). The charging current decreases approximately exponentially in this phase P2. The charging process is ended as soon as either a predetermined charging time is reached or a predetermined value of the charging current is undershot. The described charging strategy is referred to after to its characteristic phases as CC-CV charging.
During the charging process, the battery management system of the battery continuously monitors the temperatures in the battery modules which divide the battery, as well as monitoring all the cell voltages. In the event of the predetermined safety thresholds for a maximum cell temperature or a minimum or maximum cell voltage (for example Umax in FIG. 1) being undershot or exceeded, the battery management system automatically opens the high voltage contactor of the battery and thereby switches it off (to a de-energized state). This safety function is required to protect the battery against irreparable damage which in extreme cases could also lead to instability of the battery pack. The case of a raised battery temperature (above a predetermined operating temperature) is also to be avoided as far as possible during operation since it entails accelerated aging of the battery pack.
For the abovementioned reasons, also during the charging process the battery management system continuously signals the values of the cell voltages and module temperatures to a control device of a charging device which is used to charge the battery. During the charging process, the battery heats up owing to thermal power loss. In order to avoid a situation in which the battery leaves the permitted temperature range during the charging process, as soon as the battery temperature exceeds a predetermined limiting value a main control device of an electric motor vehicle which comprises the battery switches on an air-conditioning compressor.
FIG. 2a shows the time profile of a current IG and of a voltage UG in or on an overall system which comprises the battery, the air-conditioning compressor and the charging device, during the charging process of the battery. FIG. 2b shows the simultaneous profile of a charging current IB and of a charging voltage UB in or on the battery. At two points t1 and t2 there is in each case a switch-on process of the air-conditioning processor. This switch-on process requires at short notice an increase in the current IG which cannot be made available by the charging device alone but instead also has to be supplied from the battery. This results in the charging current IB briefly collapsing at both times. The control device of the charging device measures the dip in the charging current and immediately smoothes out the power loss by increasing the charging voltage UB. Since the air-conditioning compressor requires a very much lower current even shortly after the switch-on process, the battery is charged with an extremely high charging voltage UB shortly after the switch-on process (see abrupt increase in UB shortly after the times t1 and t2 in FIG. 2b). In the event of the charging process of the battery being in a phase with relatively low cell voltages (for example time t1), this is unproblematic. In contrast, in the event of the charging process of the battery being in a phase with relatively high cell voltages (for example time t2), uncontrolled switching off of the battery can occur owing to the infringement of the maximum cell voltage limit.